54 GENERAL CONDITIONS FOR GROWTH 



temperature at which the bacteria are killed within a given 

 time limit, is a matter of great practical importance in 

 many ways and numerous determinations of this have been 

 made with a great many organisms and by different observers. 

 The factors which enter into such determinations are so 

 many and so varied that unless all the conditions of the 

 experiment are given, together with the time of application, 

 the mere statements are worthless. It may be stated that 

 all young, actively growing (non-spore-containing) disease- 

 yroducing bacteria, when exposed in watery liquids and in 

 small quantities are killed at a temjMrature of 60° within half 

 an hour. It is evident that this fact has very little practical 

 application, since the conditions stated are rarely, if ever, 

 fulfilled except in laboratory experiments. (See Sterilization 

 and Pasteurization, Chapter XIII.) 



LIGHT. 



Speaking generally, it can be said that light is destructive 

 to bacteria. Many growing forms are killed in a few hours 

 when properly exposed to direct sunlight and die out in 

 several days in the diffuse daylight of a well-lighted room. 

 Even spores are destroyed in a similar manner, though the 

 exposure must be considerably longer. Certain bacteria 

 which produce colors may grow in the light, since the pig- 

 ments protect them. Some few kinds, like the sulphur bac- 

 teria, which contain a purplish-red pigment that serves 

 them to break up H2S, need light for their growth. Since 

 disease-producing bacteria are all injuriously affected by 

 light, the advantage of well-lighted habitations both for 

 men and animals is obvious. 



The destructive effect of light is due largely to the ultra- 

 violet rays. ]\Iany investigators have studied this phenome- 

 non. An excellent example of this type of work is that done 

 by George N. Burger of the University of Cincinnati.^ 



Burger spread the organisms in a thin layer and exposed 

 this to the rays from a quartz mercur}' arc. The following 

 table gives some of his results and the photograph (Fig. 51) 

 shows the effect on one of the organisms studied : 



1 Correlated Investigations in the Basic Sciences, Series IV, 1928, No. 2, 

 pp. 46-54. 



